A switching power supply is defined as a means for transforming a DC voltage, which comprises an AC transformer of which the primary winding is supplied with a DC input voltage, a switching means which periodically interrupts the primary current of the transformer, a regulation means which regulates the switching means to allow the aforementioned function, a rectifier which is connected to the secondary winding of the transformer, and a smoothing circuit which smoothes the DC output voltage of the rectifier.
Advantages inherent to a switching power supply are:
(a) A relatively large DC voltage transformation ratio is allowed, and
(b) The DC output voltage is readily regulated by regulating the duty ratio (the ratio of the conductive period vs. the switching period) of the primary current of a transformer, thereby allowing a switching power supply to produce a stable DC output voltage, regardless of variation in the input voltage and/or the load, provided a sensor for monitoring the DC output voltage is arranged and a switching means is regulated in response to this detected DC output voltage.
The simplest and most effective circuit for a switching power supply having the output capacity range of 10 through 300 W can be designed with a single transistor flyback system or a single transistor forward converter system. A schematic connection diagram of each such system is shown respectively in FIG. 1 and FIG. 2.
Referring to FIGS. 1 and 2, a switching power supply comprises a rectifier 1, which converts an AC input to a DC output, a transformer 4, which has a ferrite core, a rectifier 5, a smoothing circuit 6, and a switching means 3 which is illustrated as a switching power transistor which periodically interrupts the primary current of the transformer 4, and a regulation means 2 which further comprises a pulse oscillator, a pulse width modulation circuit and a drive circuit.
In the prior art, 20 through 30 KHz is employed as the switching frequency for switching power supplies. On the other hand, it is well-known that a higher switching frequency readily causes a smaller volume and a lighter weight for transformers and/or inductors, if any, resultantly causing a smaller volume and a lighter weight for a switching power supply. This is the reason why various efforts are being used for development of a switching power supply which allows employment of a higher switching frequency.
However, it is not easy to manufacture a switching power supply which allows employment of a high switching frequency which is in excess of 50 KHz due to various parameters.
The first parameter which disturbs increase of switching frequency of a switching power supply is the nature which is inherent to a semiconductor device more specifically to a transistor. In other words, a transistor having a higher switching speed is inevitably accompanied by a lower breakdown voltage. Namely, a switching transistor having a higher switching speed is required to have (a) a smaller relative resistance in the base region and (b) less width for the base, despite the fact that these requirements evidently result in a smaller breakdown voltage. Incidentally, a higher switching frequency employed for a switching power supply is accompained by a higher inpulse voltage, requiring a higher breakdown voltage for a switching transistor employed for a switching power supply. Thus, it is difficult to increase switching frequency of a switching power supply, unless some other means is employed to decrease the impulse voltage which may occur in the transistor circuit or the breakdown voltage required for a transistor employed in a switching power supply.
The second parameter which inhibits increase of switching frequency of a switching power supply is a limitation imposed by the switching speed of a switching power transistor employed in a switching power supply. Namely, it is the delay in turning-off a switching transistor due to the influence of storage time. In other words, a collector current continues flowing for a period which is the sum of storage time and fall time, even after a forward base current is discontinued. This means the switching speed of a transistor is limited by the length of storage time and fall time, resultantly meaning that a switching power supply requires a switching frequency which is slow enough in comparison with the sum of storage time and fall time of a transistor employed in the switching power supply, because no switching function can be expected for an excess switching frequency.
Due to these two parameters, it is not easy to manufacture a switching power supply which employs a higher switching frequency.
One example of the time chart of a base drive system generally employed for a switching power supply available in the prior art is shown in FIG. 3. Referring to the Figure, a forward base current I.sub.B1 is applied continuously during the forward half t.sub.1 ' of the switching period T, and reverse base current I.sub.B2 is applied at the beginning of the reverse half of the switching period T to turn off the collector current I.sub.C. However, even after the reverse base current I.sub.B2 has been applied, the collector current I.sub.C continues flowing during the storage time t.sub.s and fall time t.sub.f. As a result, the conductive period t.sub.1 is longer than the period in which the forward base current I.sub.B1 flows, by the sum of the storage time t.sub.s and the fall time t.sub.f.
Since the flow of the collector current I.sub.c of a transistor is limited to the period in which a current is maintained in the forward direction between a base and an emitter, the continuous supply of forward base current I.sub.B1 is essential, provided the sum of storage time t.sub.s and fall time t.sub.f is marginal in comparison with the switching period T.
However, this base drive system is involved with the following drawbacks, in the cases where the switching frequency is high in comparison with the sum of storage time t.sub.s and fall time t.sub.f of a switching power transistor employed in a switching power supply.
Firstly, since the collector current I.sub.c is not discontinued until the sum of storage time t.sub.s and fall time t.sub.f expires, this delay in turning-off of a switching power transistor becomes a parameter which disturbs an accurate maintenance of duty ratio (t.sub.1 /T) of a switching power supply. It is clear that an entirely no switching function is allowed in the extreme cases.
Secondly, a higher switching frequency increases the ratio (t.sub.s +t.sub.f /T) of the period (t.sub.s +t.sub.f) in which a switching loss (a loss generated in a transistor due to its resistance, which is gradually increased following the progress of turning-off) is generated and the entire switching period (T), resultantly causing a possibility of overheating a switching power transistor employed in a switching power supply.